Abradable seal coatings for aero-engines are required to maintain structural integrity and stable tribological performance over a wide temperature range. However, during the atmospheric plasma spraying (APS) process, a critical trade-off exists between coating densification and severe high-temperature burnout of the graphite functional phase. In this study, APS parameters are systematically optimized to achieve synergistic regulation of coating microstructure and service performance. Core-shell structured nickel-coated graphite (Ni@C) composite powders are successfully synthesized by in-situ electroless plating using nickel sulfate and hydrazine hydrate at 80 ℃. The continuous nickel shell significantly improves the powder flowability to 94.5 s/50 g and the apparent density to 0.91 g/cm³, providing favorable feedstock characteristics for thermal spraying. To establish the intrinsic relationship between spraying parameters and coating properties, an L9(34) orthogonal experimental design is employed. The effects of spray distance (80, 100, and 120 mm), gun traverse speed (200, 250, and 300 mm/s), spraying power (17.8, 24.4, and 28.0 kW), and powder feed rate (2.0, 3.3, and 4.6 g/s) on deposition efficiency, porosity, Rockwell hardness, and graphite burnout rate are systematically investigated. Range analysis and microstructural characterization are further conducted to clarify the governing mechanisms of these parameters. In addition, the high-temperature reliability of the optimized coating is evaluated through water-quench thermal shock testing, prolonged isothermal aging at 400 ℃, and friction and wear tests over a temperature range from room temperature (RT) to 500 ℃.
Range analysis shows that different process parameters predominantly control different coating properties. Powder feed rate exerts the strongest influence on deposition efficiency and Rockwell hardness, whereas gun traverse speed mainly governs coating porosity by regulating the splat overlap behavior. Spraying power is identified as the dominant factor affecting graphite burnout. Microstructural observations indicate that excessive heat input promotes particle melting, splat spreading, and matrix densification, but simultaneously intensifies graphite oxidation, leading to large burnout cavities, local collapse defects, and reduced abradability. To address this trade-off, an optimized medium-heat-input strategy is proposed. A well-structured Ni@C composite coating is obtained under the optimized parameter combination of 24.4 kW spraying power, 200 mm/s gun traverse speed, 120 mm spray distance, and 3.3 g/s powder feed rate. Under these conditions, the coating achieves a balanced microstructure, exhibiting a porosity of 12.70%, a Rockwell hardness of 28.75 HR15Y, and a significantly reduced graphite burnout rate of 18.89%.
The coating prepared under the optimized parameters exhibits excellent service performance. During thermal shock testing, it withstands 75 cycles between 400 °C and room-temperature water quenching without visible macro-cracking or delamination, demonstrating superior thermal shock resistance. Isothermal aging at 400 ℃ for 100 h reveals a three-stage hardness evolution behavior involving stress relaxation, oxidation strengthening, and dynamic equilibrium, with the hardness eventually stabilizing within a favorable abradable range of 21.5±1.0 HR15Y. Tribological tests demonstrate excellent wear resistance over a wide temperature range. From RT to 500 ℃, the average friction coefficient remains stable between 0.26 and 0.35, while the specific wear rate is maintained at a low level of 4.95 to 10.79×10-6 mm3/(N·m). The optimized medium heat input effectively balances matrix densification and graphite retention. The high-temperature tribological stability is mainly attributed to the synergistic lubrication system formed by the protective NiO oxide film and the continuous release of retained graphite. This structure effectively suppresses crack propagation and severe adhesive wear, indicating promising application potential in medium- and low-temperature gas-path sealing sections of advanced aero-engines.
Key words
Ni-coated graphite /
abradable seal coating /
plasma spraying /
orthogonal experiment /
tribological performance /
microstructure /
thermal shock resistance
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Funding
National Natural Science Foundation of China (52571111); National Key Research and Development Program of China (2022YFC3902001); Natural Science Foundation of Henan Province for Distinguished Young Scholars (252300421050)
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